1445 Introductory Astronomy I. Chapter 6 Earth and Moon R. S. Rubins Fall, 2010. The Earth 1. The Earth 2. 2005 photograph by Rosetta taken from 600,000 km. Earth and Moon. 1992 photograph by Galileo taken from 600,000 km.
Earth and Moon
R. S. Rubins Fall, 2010
2005 photograph by Rosetta taken from 600,000 km.
1992 photograph by Galileo taken from 600,000 km.
This method, described in the early 20th century by James Jeans, is analogous to the firing of a spaceship, with an initial speed fast enough for it to escape the Earth’s gravity.
For particles above about an altitude of about 500 km, the air is so dilute, that gas particles rarely collide.
At that altitude, the average speed of a hydrogen atom is about 5 km/s, which is below the escape velocity of roughly 11 km/s.
However, the H atoms have a distribution of speeds, with some of them going faster than the escape velocity.
If one such atom has a velocity directed away from the Earth’s surface, it will leave the Earth.
In this method, the upper atmosphere absorbs UV radiation from the Sun, warms and expands, pushing air upwards at ever faster speeds.
The best evidence for this escape method has come from observations of the extrasolar planet HD 209458b.
In this case, a positively charged ion, following a magnetic line, collides with a neutral atom, and steals an electron from it.
The now neutral atom is not constrained to the field line, and breaks free.
In this case, an ion follows a magnetic field line which does not return back to Earth.
The ultimate loss of the Earth’s water should occur through the breakdown of water into its components, hydrogen and oxygen.
The hydrogen would tend to move towards the upper atmosphere, leaving the oxygen behind.
After about 3.8 billion years, the remaining water should be confined to the polar regions.
In 3,8 billion years
(The height of Mt. Everest is 29,000 ft.)
Top of mesosphere
i. P waves can travel through all materials,
ii. S waves can travel only through solids.
The Jack Hills zircons in Australia are the oldest rocks known.
These peaks are formed by the rebound of the ground following a very large meteoric impact.
Rilles are lunar canyons, probably carved by lava flows.
Powdered rock (theregolith), built up over billions of years, covers the surface of the Moon. Although not moist, it sticks together underfoot like wet sand.
These pebble-like beads observed by the Apollo missions of the 1960s and 1970s were found in 2008 to contain about 46 parts per million of water, indicating that water was a part of the Moon’s early existence.
Co-creation (or double planet) hypothesis
Pro: simple, reasonable idea.
Con: the Earth’s average density (5.5 g/cm3) is much larger than that of the Moon (3.35 g/cm3).
Fission hypothesis (George Darwin, 1878)
Pro: the Moon’s density is similar to that of the Earth’s crust.
Con: no mechanism has been proposed for how such a large object could have been ejected.
Capture hypothesis (early 1900s)
Pro: the density difference is not a problem.
Con: no simple mechanism exists for how it could have happened, since capture would have required the presence of a third large object in the vicinity at the same time.
Apollo Astronauts (1969-1972) brought back Moon-rocks, which were found to be similar to the Earth’s crust, except for a smaller proportion of volatile (easily vaporized) elements, indicating that the Moon was formed at a higher temperature than the Earth.
Large impact hypothesis (ca. 1980)
i. the vaporized material would be depleted of water and
ii. the Moon, like the Earth’s crust, would be less dense than
iii. the glancing impact greatly increased the Earth’s rotation
rate, so that in its early existence, the day lasted only about
Blue is iron
Red is rocky
Model 1: Moon only, neglect friction
The water is therefore pulled away from the surface by the Moon, giving a high tide.
The Earth is therefore pulled away from the water, again giving a high tide also.
High tide Low tide
Model 2: Moon and Sun, neglecting friction
Model 3: Moon and Sun, plus friction
The laser light, bouncing off reflectors placed on the Moon by Apollo Astronauts can be used to measure the Moon’s distance to within 2 cm.